The field of quantum simulation ushers in a major breakthrough!

◎ Science and Technology Daily reporter Wu Changfeng

The reporter learned from the University of Science and Technology of China on February 7 that Science published a major breakthrough in the simulation of a large number of sub-simulations in China on February 4 - Pan Jianwei, Yao Xingcan, Chen Yuao's team based on the ultra-cold lithium-dysprosium atom quantum simulation platform, the first measurement of the second sound attenuation rate (acoustic diffusion coefficient), and accurately measured the thermal conductivity and viscosity coefficient of the system.

Video source: University of Science and Technology of China

How does heat spread? Usually by diffusion, i.e. gradually decreasing from near to far. However, in some cases it may also propagate in the form of waves, much like sound waves. Therefore, this phenomenon is called the second sound, and the relative ordinary sound waves are called the first sound.

The second sound does not appear in ordinary substances, but only in certain special substances, such as overcurrent helium. Superflow is a fluid whose viscosity becomes zero, which is a macroscopic quantum phenomenon. For example, a superfluid packed in an open cup can crawl out spontaneously. Another example is that if a vortex is produced in an ordinary liquid, it will gradually disappear, while the vortex in the superfluid will not decay and will exist forever.

By measuring the second sound and its associated thermal transport phenomena in liquid helium, a universal theory was established called kinetic scale theory. This theory has important guidance for phase transitions in many quantum systems, such as high-temperature superconductivity, because it states that the phase transition processes of many different systems follow some of the same universal functions. However, it is difficult to accurately measure these universal functions in liquid helium because of its narrow critical area and limited maneuverability. A second sound was discovered through liquid helium, but it was difficult to penetrate.

Pan Jianwei, Yao Xingcan, Chen Yuao, and others from the University of Science and Technology of China, in collaboration with Australian scientist Hu Hui, observed the critical divergence behavior of entropy wave attenuation in the Fermi superfluid at the limit of strong interaction (unit positive), revealing that there is a considerable phase transition critical zone in the system, and obtaining important transport coefficients such as thermal conductivity and viscosity coefficient. This work provides important experimental information for understanding the quantum transport phenomena of strongly interacting Fermi systems, and is an example of using quantum simulation to solve important physical problems. On February 4, the results were published in the form of a long article in the form of an international authoritative academic journal "Science".

More than 80 years ago, Landau established the two-fluid theory, successfully explained the phenomenon of superflow of helium-4 liquids (strongly interacting Bose systems), and predicted that entropy or temperature would propagate in the form of waves in the supercurrent. The nature of entropy waves is similar to that of traditional sound waves, which gradually decay during propagation, so Landau named it second sound. The propagation and attenuation of the second sound, directly coupled to the superflow sequence parameter, is a unique quantum transport phenomenon that exists only in superfluids. Studying the attenuation behavior of the second sound in the Fermi superflow can not only answer the long-standing problem of "whether the two-fluid theory can describe the low-energy physics of the strong interaction Fermi superflow", but also characterize the critical transport phenomenon of the strong interaction Fermi system at the superluid phase transition.

The superfluid formed by ultracold Fermi atoms under the strong interaction (monogram) limit has excellent purity and controllability, which brings a new opportunity to study the attenuation of the second sound, which is also an important goal in the field of ultracooled atom quantum simulation. Observing the attenuation of the second sound requires not only the preparation of high-quality density uniform Fermi superflows, but also the development of methods to detect weak temperature fluctuations.

Although Fermi superflow has been implemented for nearly 20 years, the above two key technologies have not been breakthroughs, so it is impossible to study the attenuation of the second sound.

Courtesy of University of Science and Technology of China

In this work, after more than 4 years of hard work, the research team of the University of Science and Technology of China has built a new ultra-cold lithium-dysprosium atom quantum simulation platform, integrated the development of gray sticky groups and algorithmic cooling, cassette-type photomotive well and other advanced ultra-cold atom regulation technologies, and finally successfully achieved the preparation of the world's leading uniform Fermi gas.

At the same time, based on low-noise traveling-wave optical lattice and high-resolution in situ imaging techniques, the research team experimentally realized and theoretically interpreted the Bragg spectroscopic methods of low-momentum transfer (about five percent of Fermi momentum) and high energy resolution (better than one-thousandth of Fermi energy), and used them to achieve high-resolution measurements of the density response to the system. On the basis of the above two key technological breakthroughs, the research team successfully observed the signal of the second sound in the density response of the monoframic superfluid, and obtained a complete density response spectrum of the monofumfermi superfluid, and the experimental results were highly consistent with the description based on the theory of dissipative two fluids.

Further, the research team obtained the attenuation rate (acoustic diffusion coefficient) of the second sound, and accurately determined the thermal conductivity and viscosity coefficient of the system. The results show that the transport coefficients of the monoframic Fermi superfluid have reached the universal quantum mechanical limit.

In addition, they observed the critical divergence behavior of the above transport near the superluid phase transition and found that the monofumer superfluid has a considerable critical region (about 100 times more than the liquid helium superfluid critical region). This finding lays the foundation for further quantum simulation studies using the system to understand anomalous transport phenomena in the strongly correlated Fermi system.

The Science reviewer spoke highly of the work, saying it "showcases a masterpiece of amazing experiments," "an excellent paper," and "promises to be a milestone in the field of quantum simulation."

Source: Science and Technology Daily Cover image courtesy of Visual China

Editor: Wang Yu

Review: Julie

Final Judge: He Yi